The process of manufacturing integrated circuits uses masks to create the circuit on silicon. The masks are only created once for each chip design, but must be highly accurate. Traditional mask writing methods typically offer a tradeoff between speed and accuracy. The process of manufacturing the mask usually employs techniques that give the highest possible accuracy, at the cost of slower mask creation. Techniques that allow faster mask creation are often less useful since they result in significantly less accurate masks.
For example, multi-pass writing is a technique commonly used in the generation of IC masks to get higher accuracy at the cost of writing time. This technique averages out the errors made in writing each figure. A typical mask writing machine, such as the Toshiba/NuFlare EBM-4000, only reaches its specified accuracy when the writing is done in 4 independent passes. Typically 4 pass writing is twice as accurate as single pass writing due to the statistical averaging of errors (in this case the error goes down as the square root of the number of passes). However, 4 pass writing takes longer (almost 4 times as long) to write a mask. Raster-based machines offer similar tradeoffs, with large spots yielding faster writing but less accuracy, and small spots giving the best accuracy but the longest writing times.
This extra time translates directly to mask cost. In the past, this tradeoff was acceptable since mask cost was not (in general) a significant expense compared to other project expenses. As the number of features on a chip, and their required accuracy, increases, the mask costs are becoming very significant indeed. A way to write the masks faster would be very useful in reducing the costs. Currently, however, conventional methods of doing this require relaxing the accuracy requirement, resulting in a mask that is less accurate. The resulting chip has a lower yield, and hence is not desirable. In conventional approaches, either all features were written to a single accuracy standard which resulted in a slower writing process, or a special database had to be provided to specify which items were more or less critical, but not why, thus limiting possible optimizations.
So currently customers using this approach select between fast writing, which gives lower mask costs, but also lower yield production, and slow mask writing, which gives the best possible yield but at the cost of very slow to produce and expensive masks.
Another approach uses a device for coupling design intent and OPC (Optical Proximity Correction). The design intent is used to write fewer mask features, with the same overall goal of minimizing mask cost. These proposals concentrate on writing fewer figures and do not consider the possibility of writing the same number of mask features, but writing the less-critical features with a faster but less accurate method.